Inertia And KSP

[NeoMorph]

One thing that always bugs me about KSP is the lack of inertia. Multi-ton craft behave like they have the inertia of a wet egg carton. No, strike that. They have the inertia of a DRY egg carton. It's what ruins the feeling of immersion for me.

I've tried to work around it by reducing the strength of the reaction wheels and reduced the thrust of the RCS but when you bump anything it still bounces all over the place.

Sooooo... is there any mod out there that can increase inertia in relation to weight mass of your craft?

 

Edit: Ooops, my mistake... I meant to say mass instead of weight.

Edited July 1, 2016 by NeoMorph

[Bill Phil]

What exactly do you mean? Inertia is the resistance of moving that objects have. It's still there in KSP. The engines are just enormously overpowered. You want a good fix, huh? I don't know of any mods, but you could add more mass to your ships.

Hope that helps.

[NeoMorph]

8 minutes ago, Bill Phil said:

What exactly do you mean? Inertia is the resistance of moving that objects have. It's still there in KSP. The engines are just enormously overpowered. You want a good fix, huh? I don't know of any mods, but you could add more mass to your ships.

Hope that helps.

Yup... have you never just bumped a craft and caused it to spin like crazy. Or even just bumped a large craft with a small one and the big one doesn't even resist. It's like it's totally empty. Granted SOME effect would show (like the problem with the exercise cycles in orbit caused the station to move so they had to work out a way to counter it) but the sheer effect of large craft in KSP just don't show the inertia stopping them moving when you gently bump them with a smaller craft. It's like if you run into a car with a push bike. In KSP the car would behave like it's also a push bike.

 

[ferrer]

The way things behave in KSP is the same as the way they would behave in real life. Maybe you're subconsciously expecting a resistance force when you push something in KSP, but there isn't one since you're in a vacuum?

[Guest]

Just now, ferrer said:

The way things behave in KSP is the same as the way they would behave in real life. Maybe you're subconsciously expecting a resistance force when you push something in KSP, but there isn't one since you're in a vacuum?

This exactly.  There is nothing to stop the larger craft from moving.  Smaller craft imparts a force, the larger craft moves, but in accordance with Newton has nothing to stop it moving.  Large craft react less to an impact than a smaller craft due to mass but they will still move.

[NeoMorph]

No, what I'm saying is if you bump a huge craft it seems to move as much as if you bump a large craft it seems to move the same as if it was a small craft.  The large craft should move but a lot less than the small craft due to the higher inertia of the larger craft.

[NeoMorph]

Hang two 2ltr pop bottles on strings. Fill the one with water and leave the other one empty. Now put another bottle on a string and half fill it with water. Swing it at the full bottle... then swing it from the same place at the empty one. The Empty one will ping off while the full bottle with just move a little. THAT is not what I see in KSP is what I am saying. Granted that if that happened in space there would be damage like the MIR/Progress collision but in that case the Progress went flying off while MIR just started to roll (which screwed up the solar panels alignment).

Perhaps my synaesthesia is acting up again... it does occasionally.... or more likely I have a phantom force problem. Hate it when that happens.

[stormdot5]

I think you might have a problem with your frame of reference. Watching the two objects from a third position in real life shows you the difference between the two after a collision. In KSP you are locked onto one of the objects, and only the other object appears to move. This might skew your observations.

Place two craft in a high circular orbit so that their speeds are constant, then collide them at a known collision speed. Calculate the change in velocity on each craft taking mass into account and it should come out correctly.

[NeoMorph]

Hmmm... you may be right... maybe it's that NASA tends to drive like a granny heh.

Edited July 2, 2016 by NeoMorph

[Corona688]

We like to put docking couplers on the ends of our space stations because it looks nice and is convenient to reach -- but that's the worst place to put a docking port, mechanically speaking.  Anything that hits off-angle gets a huge mechanical advantage, the same way a wrench does.  The farther your station's docking port is from its center of mass, the worse it'll be tumbled by an impact against it.

The spacecraft you're flying, on the other hand, gets bumped in line with its center of mass and tumbles much less, and has the gyro capacity to absorb that in moments anyway.

Try bumping into your station's center of mass and see how it reacts.

(Also remember that empty tanks and habitable sections are made of marshmallow while command pods are mostly solid metal with a tiny amount of meat.)

Edited July 2, 2016 by Corona688

[ferrer]

2 hours ago, NeoMorph said:

No, what I'm saying is if you bump a huge craft it seems to move as much as if you bump a large craft it seems to move the same as if it was a small craft.  The large craft should move but a lot less than the small craft due to the higher inertia of the larger craft.

I think it only seems like this because your camera stays on the small craft the whole time. It might seem like the large craft is being pushed away, but the reality is that the small craft is just rebounding, and the large craft's velocity isn't changed much.

[Tyko]

17 hours ago, NeoMorph said:

Hang two 2ltr pop bottles on strings. Fill the one with water and leave the other one empty. Now put another bottle on a string and half fill it with water. Swing it at the full bottle... then swing it from the same place at the empty one. The Empty one will ping off while the full bottle with just move a little. THAT is not what I see in KSP is what I am saying. Granted that if that happened in space there would be damage like the MIR/Progress collision but in that case the Progress went flying off while MIR just started to roll (which screwed up the solar panels alignment).

Agreed Neo...if a kerbal in a space suit bounces off of a space station, or kicks a lander leg he/she does impart some force onto the larger object, but the amount of movement the larger craft exhibits doesn't seem to reflect the relative mass of the two objects. Often it seems like the lander/station moves a lot more than a multi-ton object would when a Kerbal hits it.

[Snark]

16 hours ago, ferrer said:

I think it only seems like this because your camera stays on the small craft the whole time. It might seem like the large craft is being pushed away, but the reality is that the small craft is just rebounding, and the large craft's velocity isn't changed much.

^ This.

The only way you can really see what's happening in a collision between two objects is if your camera is attached to a third nearby craft that isn't participating in the collision and therefore is remaining stationary.  If you're controlling one of the two ships involved, it will always seem like the other ship is flying away rapidly while your own just sits there.

KSP models inertia quite well.  It gets it for free from the Unity physics engine.  The game sets up the objects, gives them the appropriate geometry and mass, supplies angular moment tensors, and then just sits back and lets the physics engine do the work.

[Tyko]

On 7/2/2016 at 11:24 AM, Snark said:

KSP models inertia quite well.  It gets it for free from the Unity physics engine.  The game sets up the objects, gives them the appropriate geometry and mass, supplies angular moment tensors, and then just sits back and lets the physics engine do the work.

I've watched a Kerbal kick over a multi-ton lander. Just because there's low gravity doesn't mean that lander has no inertia. A kerbal shouldn't be able to kick one over.

[Snark]

10 minutes ago, tjt said:

I've watched a Kerbal kick over a multi-ton lander. Just because there's low gravity doesn't mean that lander has no inertia. A kerbal shouldn't be able to kick one over.

Well, it's hard for me to say whether a Kerbal "should" be able to kick one over or not, without seeing a video.  However, even if we grant that what you observed was "unreasonable" ... that still doesn't mean that "inertia is wrong".  It's pretty clear that KSP models inertia just fine.  It can't help doing so-- the physics engine provides that.

It may be that the kerbal is modeled unrealistically (though I'm not sure how germane the term "realistic" is, when we're talking about imagininary little green space aliens)-- e.g. that they have freakishly high strength, or a freakishly high coefficient of friction between their boots and the ground.  Have never tested to check.

However, I think the modeling of inertia is just fine.

[Tyko]

Just now, Snark said:

It may be that the kerbal is modeled unrealistically-- e.g. that they have freakishly high strength, or a freakishly high coefficient of friction between their boots and the ground.  Have never tested to check.

hmmm..fair point that I can't pinpoint the inertia modeling...I have to say though, that as a player that sees a kerbal kick a multi-ton lander and then to have the lander sail into the air and flip over I'll say "there's something wrong here" 

It may not specifically be the inertia model, but the user perception is that the lander is behaving like it has really low mass.

[Snark]

1 minute ago, tjt said:

hmmm..fair point that I can't pinpoint the inertia modeling...I have to say though, that as a player that sees a kerbal kick a multi-ton lander and then to have the lander sail into the air and flip over I'll say "there's something wrong here" 

It may not specifically be the inertia model, but the user perception is that the lander is behaving like it has really low mass.

Well, sure.  If the assertion is "there are some physically quirky things about the way kerbals are modeled in-game", then I'm totally on board with that. 

But that's a different thing than saying that inertia is modeled wrong, which is what this thread has been discussing.

[Tyko]

Again, fair point. I don't know enough about the inner workings of the code to identify which part is wonky. From a player's PoV, when a huge craft goes flipping sideways like it's made of tin foil rather than tons of steel the perception is that the object is acting as if it's very low mass.

You're absolutely right that I can't blame the inertia code specifically.

thanks for explaining that 

[Mastikator]

1 hour ago, tjt said:

I've watched a Kerbal kick over a multi-ton lander. Just because there's low gravity doesn't mean that lander has no inertia. A kerbal shouldn't be able to kick one over.

That's the kraken in action. F = MA are the rules of KSP